Soldering is generally used for making mechanical, electromechanical, or electronic connections to a variety of articles. The distinction between expected functions of the joints is important because each application has its own specific requirements for surface preparation. Of the three soldering applications, making electronic connections is the most demanding.
In the manufacture of electronic equipment utilizing printed circuits, connections of electronic components to the printed circuits are made by soldering of the leads of the components to the through-holes, surrounding pads, lands and other points of connection (collectively, “Areas of Connection”). Typically the connection occurs by various soldering techniques.
To facilitate this soldering operation, the printed circuit fabricator is required to arrange that the through-holes, pads, lands and other points of connection are receptive to the subsequent soldering processes. Thus these surfaces must be readily wettable by the solder and permit an integral conductive connection with the leads or surfaces of the electronic components. Because of these needs, printed circuit fabricators have devised various methods of preserving and enhancing the solderability of surfaces.
One means of arranging good solderability of the surfaces in question is to provide the surfaces with a pre-coating of solder. This is typically performed by a process called hot air solder leveling or through some type of plating process. In printed circuit fabrication, however, this method has several drawbacks. The use of hot air solder leveling may cause an unacceptably high rate of defects due to shorts, particularly when dealing with small circuits. If plating is used, since it is not easy to selectively provide these areas with solder, all conductive areas of the board must be solder plated causing severe problems with the subsequent application of solder mask. In addition the foregoing processes are inefficient and relatively expensive.
Another means of arranging good solderability of these surfaces is to plate them with a final finish coating of a precious metal such as gold, palladium or rhodium. U.S. Pat. No. 5,235,139 (Bengston, et. al.), the teachings of which are incorporated herein by reference, proposes a method for achieving this precious metal final finish. Bengston et. al. propose plating the copper areas to be soldered with electroless nickel-boron, followed by a precious metal coating such as gold. See also U.S. Pat. No. 4,940,181 to Juskey, Jr. et al., the teachings of which are incorporated herein by reference for a similar process which teaches the plating of electroless copper, followed by electrolytic copper, followed by nickel, followed by gold as a solderable surface. These processes work well but are time consuming and expensive.
Various attempts have been made to selectively apply solder to the necessary areas only. One such method involves use of organic etch resists over the solder plated areas of connection followed by selective stripping of tin-lead from the copper traces before application of the solder mask. See U.S. Pat. No. 4,978,423 to Durnwith et. al. See also U.S. Pat. No. 5,160,579 to Larson, the teachings of which are incorporated herein by reference, for other known selective solder processes.
Soldering directly to copper surfaces has been difficult and inconsistent. These problems are due mainly to the inability of keeping the copper surfaces clean and free of oxidation throughout the soldering operation. Various organic treatments have been developed to preserve copper surfaces in a readily solderable state. For example, see U.S. Pat. No. 5,173,130 (Kinoshita) which teaches the use of certain 2-alkylbenzimidazoles as copper pre-fluxes to preserve the solderability of the copper surfaces. Treatments such as those taught in Kinoshita have proven successful but there is still need to improve the reliability of the process.
The method of preserving solderability proposed herein is the coating of copper surfaces to be soldered with a silver plate prior to soldering. It has been found, however, that when the foregoing method is used the silver coating has a tendency to corrode, especially if exposed to environments containing sulfur compounds. This corrosion may continue at interfaces even after the soldering is complete thereby weakening the solder connection.
It is an object of this invention to propose a method for preserving and enhancing the solderability of copper surfaces by plating said copper surface with an immersion silver plate followed by a post-treatment, thereby making the silver plate more resistant to corrosion than prior art immersion silver deposits and ensuring a strong, long lasting solder bond.